A discussion follows here below of the techniques of the prior art with reference to the particular case of MIMO-OFDM systems, i.e. multiple-input and multiple-output multi-antenna communication systems implementing orthogonal frequency division multiplexing (OFDM).
MIMO-OFDM systems can be used especially to implement iterative reception techniques which, as and when the iterations are done, improve the quality of the estimation of the sent signal as a function of the received signal.
As shall be seen here below, these techniques rely on a successive and repeated implementation of elementary modules so that these different modules exchange information on the reliability of the operation performed.
There are very many known techniques of iterative reception for multiple antennas systems in transmission and/or in reception.
These iterative techniques include especially:                receivers implementing an maximum likelihood (ML) type algorithm which have the drawback of inducing high processing complexity;        receivers based on linear filters such as those proposed by M. Sellathurai and S. Haykin in Turbo-blast for wireless communications: theory and experiments, IEEE Transactions on Signal Processing, Vol. 50, No. 10, pp. 2538-2546, 2002. These receivers rely on MMSE (minimum mean square error) type filtering techniques and interference cancellation. They have the advantage, as compared to maximum likelihood receivers, of being far less complex.        
Certain of the algorithms implemented in receivers based on linear filters, and this is the case of an embodiment of the present invention, rely on an approximation in which the estimation of the symbols is considered to be perfect as of the second equalization, for the computation of the coefficients of the equalization filter (also called a total equalization block here below) which are then computed only twice for each block.
Referring to FIG. 1, a description is now made of the classic functional sequence of a transmission device 100 (commonly called a transmitter) of a source signal 10 formed by binary elements in a MIMO-OFDM type transmission context.
The (binary) source signal 10 to be sent undergoes a channel encoding CC11 and then a Π interleaving 12. It then goes through a “mapping module M, 13 intended to convert binary elements or bits into complex symbols: a module of this kind thus associates a group of bits with a complex symbol belonging to a constellation (of a QPSK, 16 QAM or other type). The sequence of symbols output from the mapping module M 13 is commonly called an M-ary signal. A space-time block encoding 14 is then performed for each group of Q symbols which are then modulated in blocks 151, 152 to 15Nt according to an OFDM type multi-carrier modulation technique, and then sent on Nt transmission antennas 161, 162 to 16Nt.
As illustrated in FIG. 2, the classic functional chain of a reception device 200 (commonly called a receiver) of a signal sent by the above-mentioned transmission device 100 has two stages, namely a “space-time decoder” 24 (i.e. a converter of symbols into bits) and a channel decoder 26 which exchanges extrinsic or a posteriori information in an iterative loop until the receiver converges. These stages may be separated especially by an interleaver 22i, used to decorrelate the outputs and then give them to the next decoding stage. In other words, the information exchanged will be decorrelated from one stage to another.
Thus, a signal r is received on NR receive antennas referenced 251 to 25NR, then demodulated into blocks 271, 272 to 27NR, according to a demodulation technique which is the reverse of the multi-carrier technique implemented at transmission Each receive antenna 251 to 25NR receives a linear combination of the symbols sent on each of the Nt transmit antennas. The first decoding stage 24 comprises a first space-time linear decoding block 20 (here below also called a total equalization block) according to a criterion, for example an MMSE or “zero forcing>> (ZF) type criterion. The equalized signal {tilde over (s)}(p) output from the space-time decoding block 20 is then fed into a “demapping” module M−1 231, and then undergoes a de-interleaving operation Π−1 221 and then a channel decoding CC−1 21. At output of the second channel decoding stage 26, an estimated flexible binary signal {circumflex over (d)} is obtained on the encoded bits (it may be recalled that the bits used in the iterative process are called “flexible” because their value depends on the probability of the bits).
Since the method is iterative, this estimated flexible binary signal {circumflex over (d)} is subjected to another interleaving Π 222 and another “mapping” M 232, in order to obtain an estimated M-ary signal ŝ(p) that can be re-injected into the space-time decoding block 20 for a following iteration to improve the estimation of the signal received.
At the first iteration, the receiver carries out a classic equalization of the received signal because no estimated signal is available. At the following iterations on the contrary, the previously estimated symbols are used by the equalizer to cancel one or more interferences affecting the received signal.
Classically, receivers based on linear filters implement interference-cancellation mechanisms that rely on the use of blocks of symbols.
According to this iterative reception technique, it is necessary for all the bits that form the symbols of a space-time block to be decoded in order to subtract the interferences affecting the symbols which form the received signal.
More specifically (referring to FIG. 2), the “demapping” module M−1 231, which works symbol by symbol receives a block 201 of equalized symbols formed by Q equalized symbols (corresponding to the equalized symbol {tilde over (s)}(p)) and delivers packets 202 of nb converted bits.
The de-interleaving module Π−1 221 processes all the nb converted bits, bit block by bit block (the bit block having a size equal to the de-interleaving size of the module 221), and delivers blocks 203 of de-interleaved blocks.
The channel decoding module CC−1 21 receives the blocks 203 of de-interleaved bits and, after a certain latency, delivers blocks 204 of decoded bits formed by one or more bits.
The interleaving module Π−1 222 processes all the blocks 204 of decoded bits, bit block by bit block (the bit block having a size equal to the interleaving size of the module 222), and delivers blocks 205 of interleaved bits.
Then, the mapping module M 232, which works by packets of nb bits, receives the blocks 205 of interleaved bits and delivers estimated symbols 206.
Finally, the total equalization block 20 waits to have Q estimated symbols at input to determine a block of equalized symbols improved by interference cancellation.
Referring now to FIG. 3, a description shall be provided of the classic functional chain of an MMSE-IC (MMSE interference canceller according to an MEQM or mean quadratic error minimization criterion) implemented by the above-mentioned reception device 200.
For the sake of clarity, the following notations will be used here below in the document:                H designates a matrix representing the transmission channel of the sent signal 10;        I designates an identity matrix sized Q×Q;        σ2=1/SNR is the variance of the equivalent noise, also equal to the reverse of the mean signal-to-noise ratio (SNR) observed on each receive antenna; and        G=HH·H is a total equalization matrix.        
The interference cancellation mechanism 300 receives the estimated M-ary signal ŝ(p) and the received signal r at input. It performs the following operations:                adapted filtering 302 of the received signal r by application of the transconjugate matrix of the channel HH delivering a filtered signal;        creation 301 of the interference from an estimated M-ary signal ŝ(p) by left multiplication of this estimated M-ary signal by an interference matrix J=G−diag(G). More generally, this interference matrix must at least take account of the channel matrix H;        subtraction of the interferences obtained at output of the block referenced 301 from the filtered signal obtained at output of the filtering block 302 to obtain an improved signal;        equalization 303 of the improved signal delivering an equalized M-ary signal {tilde over (s)}(p) by application of the matrix (diag (G)+σ2I)1. More generally, this equalization matrix must at least take account of the channel matrix H.        
Thus, according to this technique of iterative reception, a cancellation of interferences is done after the complete estimation of a block of symbols.
One drawback of this prior art technique is that it cannot be used for optimal management of the computing power of the receiver because of the sequential execution of the different elementary modules of the iterative loop. Indeed, a sequencing of this kind consists of the computation, one by one, of the estimated symbols and the rebuilding as and when an estimated symbol becomes available, of a block of estimated symbols to carry out the cancellation of the interference. In other words, for a given block of equalized symbols, it is necessary to await the estimation of all the symbols to execute the operations relating to the cancellation of interference. These operations are therefore distributed non-uniformly in that the cancellation of interference can only be done after retrieval of all the decoded and then re-interleaved bits of the space-time block.
The inventors have furthermore noted that the sequential management of the elementary modules of a receiver may prompt an increase in the processing time at reception when the interference cancellation operations are processed in series or an increase in the complexity of the receiver when they are processed in parallel.